Fixes for latest nightly docs (WIP)

Wanted to give you a heads up that I'm working on updating this generator for the nightlies (though it's not quite ready yet). I know Dash ships with Rust documentation now, but unfortunately they become out of date rather quickly since they are only updated when Rust puts out a new official release. A number of changes were made to `rustdoc` since this repository was updated, and these fixes should reflect those.

Things left to do include:

* Need to bundle and replace references to the online assets (fonts, logo) referenced in the Rust documentation.
* I'd like to update the mapping of rustdoc item types to Dash item types, since I think Dash has added some more appropriate ones (like the Variant type, as one example).
* More (all?) item names should be scoped to their crate and module, as the official Dash docset does.
* Maybe the name of the docset should be changed to differentiate from the one shipped with Dash.